I have an isolated theme and from this theme users are going to make deploys importing this theme as a npm package. The intention is when we update the theme, it would also update in users deploys, but this just happens when users trigger a deploy cleaning cache or update package.json.

Can I decrease cache time for this specific dependency? If I can’t, using it as a git submodule would solve it?

You can’t decrease the cache time, but doing so wouldn’ help in this case. If your theme is an npm package then the user would have to update their package.json in order to download the new theme. You also can’t control how other people trigger deploys on their websites, only they can. Can you clarify what your intention is and why you need to update other users themes rather than having them update the theme when they choose to? It sounds like an antipattern, but without more details I can’t say.